The present invention relates to an insulating tape for fabricating an insulating sleeve for electric conductors and intended to be impregnated with a thermo-setting epoxy resin/acid anhydride hardener mixture, particularly for the winding bars or coils of electric machines. The insulating tape consists of one or more layers of a planar, breakdown-proof inorganic material, such as mica flakes or thin mica films, which are applied to a flexible substrate and which are cemented to the substrate and to each other (and optionally, to a final cover layer) by means of a binder which contains an accelerator for promoting the hardening reaction of the resin mixture to be impregnated, wherein the binder-accelerator mixture forms a system which is self-hardening at the setting temperature of the impregnating resin.
Such an insulating tape, as is known from U.S. Pat. No. 3,647,611, makes it possible to carry out the subsequent impregnation economically with a slow-reacting impregnating resin mixture which is not subject to increases in viscosity even at the high impregnating temperatures, thereby ensuring thorough impregnation of the insulating sleeve. This is necessary because only a small percentage of the epoxy resin/acid anhydride mixture used as the impregnating resin penetrates into the insulating sleeve, while the rest must be re-used for other impregnations. The insulating tape, therefore, contains an accelerator which promotes the setting reaction of the impregnating resin mixture, so that the impregnating resin penetrated into the insulating sleeve sets in an economically justifiable time.
In order that the insulating tapes can be stored for a long time before the impregnation, the binder-accelerator mixture in the known insulating tape is chosen so that at room temperature practically no setting of the mixture occurs. Moreover, the binder-accelerator mixture forms a self-hardening system which self-hardens, however, only at the setting temperatures of the impregnating resin, which is substantially above room temperature. In this manner, it is prevented that binder which has penetrated between large-area layers of the breakdown-proof inorganic material, and thus cannot be resorbed completely by the impregnating resin, remains unhardened in the insulation.
In the known insulating tapes, cycloaliphatic epoxy resins are utilized as the binder, in which the epoxy groups are produced by the addition of oxygen to ring-located double bonds and which exhibit no reaction, or only a greatly delayed reaction, with amine hardeners or accelerators, as is generally known. With the addition of the epoxy resin/acid anhydride impregnating resin, however, these accelerators develop their full catalytic activity.
Cycloaliphatic epoxy resins, however, can be produced only with difficulty and have the further disadvantage that, according to recent investigations, these compounds are physiologically not without risk. Hence, it would be desirable to employ other epoxy resins as the binder. However, the properties of cycloaliphatic epoxy resins which render them so desirable for use in the known method, i.e., the ability to form mixtures with amine hardeners or accelerators, which mixtures do not react until in the presence of the epoxy resin/acid anhydride impregnating resin, are not exhibited by, e.g., glycidyl ether, glycidyl ester or N-glycidyl-epoxy resins.